Mishnah 18b and 19
In Mishnah 19 we find an explanation of the power that lies within the number one, for love is union, and unity endures.
Something that consists of parts, but is an integral whole, has no weakness from within, and no vulnerable seam where destruction forces could take hold. Therefore a solid entity will endure, because there is no factor, internal or external, that will bring it to an end.
Love that is purely union with another, with no element of gain to be had from the relationship, is just such an entity. It has the internal strength of integrity, and it is immune to external attack. Such love endures forever.